“Racer and the Jailbird” is about a rich young Porsche driver and the bank robber she falls for.

It’s a torrid love affair, all-consuming even though the rising-star racer suspects her man lies, and her man is getting too old to do this dangerous work he doesn’t tell her about.

“One last job,” as we say in English, and here they say it in French, sometimes in Flemish. It’s a Belgian film, after all.

And then we see that “one last job,” and the movie becomes something else, in the broader sense, and somewhat less interesting. You undestand why the title is such a contortion in English. “Le Fidele,” “The Loyal One,” wasn’t going to sell many tickets in North America. But giving it a title that makes promises the picture doesn’t keep, and is silly to boot, didn’t help.

Matthias Schoenaerts is Gino, “Gigi” to his mates. He’s been in trouble with the law since childhood, and the crowd he’s run with is the same. He may pass himself off as a car importer, but pushing 40, Gigi isn’t fooling many.

Did I mention he has a morbid, not-quite-irrational fear of dogs? “Foreshadowing” in French is “presage,”mon amis.

Gigi makes a brazen play for Benedicte, “Bibi” to her friends. They’re all hanging out at the track and the very young woman (Adèle Exarchopoulos) in that racing suit is quite fetching out of it.

She’s got to be half his age (Matthias is 41, Adele 25) and doesn’t seriously question where his money comes from. Her money comes from Daddy, and he (Eric De Staercke) is onto this guy. But he’s shockingly non-judgmental about it.

“Real men don’t lie,” is all he’ll say about it. And Gigi takes that to heart. Too late to stop that “one last job,” but there you go.

Co-writer/director Michaël R. Roskam, who did the terrific Brooklyn mob thriller “The Drop,” spends a lot of time on the romance, how Bibi scares Gigi when she’s driving, how Gigi keeps taking trips to “Poland” to explain his absences. Roskam spends just enough on “the gang” — older guys, a punk first-gen Arab, etc. — to set up The Last Big Score.

And then it happens and the movie shifts in tone, template and tempo as it morphs into a “Wait for me” prison romance. Aside from depicting the most humane prison system ever committed to film, there’s not much to this second half melodrama — some tears, some conjugal visits, life’s cruel jokes.

Worst of all, Roskam lets it go on and on. Every moment you think this heist picture/soapy romance is about to make a graceful exit passes, and MORE story, more incidents and DOGS show up.

The Belgian hunk Schoenarts (“Far from The Madding Crowd”) was in “The Drop,” because whatever women see in his smoldering good looks, he makes a passable hoodlum. Exarchaopoulus doesn’t have to suggest much more than youth and lust for her man.

We get it.

But that title, that come-on, that promise-not-kept (no getaway scenes, limited action) would be a disappointment even if the picture was better, more brisk. “Le Fidele” may sound mawkish, but it’s more honest.

You translate it to “Racer and the Jailbird” we expect more racing, more heists, more jail and more heat. “Fidele” loyally never quite clears “lukewarm,” and is awfully slow coming to a boil.

MPAA Rating: R for some strong sexuality, nudity, violence, and for language